Blender setup for orthographic image rendering
As you have many questions, here are many answers:

Position

In orthographic mode, it is not important, how far away the camera is from the object (but stay within the clipping bounds). The position parallel to the object should be set and tweaked as you wish.

Tilt

As you already calculated right, the tilt of the camera downwards is $\alpha \approx 41.4°$. Here's how to calculate it for the orthographical view:

$\cos\alpha = \frac{48\text{ pixels}}{64\text{ pixels}} = \frac{3}{4} \implies\alpha = \cos^{-1}(0.75)\approx 41.4096221°$

the other rotation axes should stay untouched (parallel to the grid)

Orthographical Size &amp; Resolution

This depends on how many grid squares you want to have in view. So we define, you want to have $m$ squares vertically and $n$ horizontally in view of your render.

If $n\ge m$ set the orthographical size to $n$.

The Render dimensions resolution: X-value is $n\cdot64$ px; Y-Value is $m\cdot48$ px.



The whole algorithm also works for different values $p,q\in\mathbb{N}$ with $p\ge q$ instead of $p=64$ Pixels and $q=48$ Pixels



With the above explained setup I got this render:





EDIT:

Fire Lancer pointed out that he wants a tileable setup.
Here's the setup with the blend file: